A battery is an electrochemical device comprising an anode, a cathode, a separator sandwiched between the anode and cathode, and an electrolyte wetting the separator and in ionic communication between the anode and the cathode. Various chemistries in which the electrochemical potential between various materials is used to generate electricity have been studied and commercially implemented. See, in general: Besenhard, J. O., Ed., Handbook of Battery Materials, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999; and Linden, D., Ed., Handbook of Batteries, Second Edition, McGraw Hill Inc., New York, N.Y., 199, both of which are incorporated herein by reference.
A battery separator is used to separate the battery's positive and negative electrodes, and is typically microporous to allow the through passage of ions from the positive and negative electrodes. In lead/acid storage batteries, either automotive or industrial batteries, the battery separator is typically a microporous polyethylene separator having a back web and a plurality of ribs standing on the back web. See: Besenhard, J. O., Editor, Handbook of Battery Materials, Wiley-VCH Verlag GmbH, Weinheim, Germany (1999), Chapter 9, pp. 245-292. The separators for automotive batteries are typically made in continuous lengths and rolled, subsequently folded, and sealed along its edges to form pouches that receive the electrodes for the batteries. The separators for industrial (traction) batteries are typically cut to a size about the same as an electrode plate.
The use of porous membrane substrates as separators within lead-acid batteries is known in the prior art. Generally separators are passive and do not directly participate in the electrochemical processes of electricity storage. Separators maintain a physical distance between the positive and negative electrodes in lead-acid batteries. Separators also permit an ionic current with as little hindrance as possible. The unhindered ionic charge transfer requires many open pores of the smallest possible diameter to prevent electronic bridging by deposition of metallic particles (i.e. lead) floating in the electrolyte.
Separators generally have a uniform thickness and a uniform pore distribution. The pore distribution ensures an overall uniform current density during operation, achieving a uniform charging and discharging of the electrodes and maximum battery efficiency. As manufacturers seek to lower production costs, separators have been made thinner, resulting in the separator becoming more flexible. The increase in flexibility may result in battery failures due to internal flexing as a battery is assembled, especially along the folding edges of a pocket separator. The increased flexibility may also cause failure due to internal or external vibrations causing the separator to deform. Lead-acid battery separators generally incorporate ribs running in the MD direction to stiffen the separator. Lead-acid battery separators may also incorporate ribs in the CMD direction. However, the ribs often provide inadequate stiffness to the separator in the MD direction, the CMD direction, or a combination thereof. Hence, a need exists for a lead-acid battery separator with enhanced stiffness.